Erythropoietin is a hormone produced by the kidneys to promote the formation of red blood cells by the bone marrow. Red blood cells are able to transport oxygen to the areas of the body that oxygen is required. However, as kidney function declines, the production of erythropoietin decreases. This results in a lack of red blood cells being produced and decreases the amount of oxygen being carried throughout the body. This condition is known as anemia. In order for people with chronic kidney disease to prevent anemia from occurring, EPO drugs and iron replacement therapy are used to enhance the red blood cell production.